Page 135 - Physics Coursebook 2015 (A level)

Chapter 8: Electric fields
QUESTIONS
11 We are used to experiencing accelerations that are usually less than 10 m s−2. For example, when we
fall, our acceleration is about 9.81 m s−2. When a car turns a corner sharply at speed, its acceleration is unlikely to be more than 5 m s−2. However, if you were an electron, you would be used to experiencing much greater accelerations than this. Calculate the acceleration of an electron in a television tube where the electric field strength is 50 000 V cm−1.
(Electron charge −e = −1.6 × 10−19 C; electron mass me = 9.11 × 10−31 kg.)
12 a
Use a diagram to explain how the electric force on a charged particle could be used to separate a beam of electrons (e−) and positrons (e+) into two separate beams. (A positron is a positively charged particle that has the same mass as an electron but opposite charge. Positron–electron pairs are often produced in collisions in a particle accelerator.)
b Explain how this effect could be used to separate ions that have different masses and charges.
Summary
■■ An electric field is a field of force, created by electric charges, and can be represented by electric field lines.
■■ The strength of the field is the force acting per unit
■ positive charge at a point in the field, E = QF .
■■ In a uniform field (e.g. between two parallel charged plates), the force on a charge is the same at all points;
the strength of the field is given by E = − dV .
■■ An electric charge moving initially at right-angles to a uniform electric field follows a parabolic path.
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